1. Field of the Invention
The present invention relates to a method of producing a micromechanical structure for a micro-electromechanical element, and, more specifically, to a method of producing a diaphragm-like structure in a semiconductor wafer in which an integrated circuit is to be produced later on making use of conventional standard semiconductor processes.
2. Description of Prior Art
Due to the fast development in the field of semiconductor industry and microelectronics, micromechanical elements, e.g. silicon-based micromechanical pressure measurement cells, replace classical mechanical pressure transducers more and more. Micromechanical elements are used in great amounts e.g. in the fields of automation technology and medical engineering as well as in automotive vehicles. The systems preferably used in this connection are micro-electromechanical integrated systems which realize the combination of mechanical and electronic functions on one substrate. In addition to the electronic components produced in CMOS or similar technologies, which are e.g. measuring transducers, amplifiers, storage means, microcontrollers, etc., mechanical components exist in the same layers. These mechanical components may e.g. be diaphragms of pressure sensors, elastic sheets of valves or pumps, oscillating masses of acceleration sensors, movable fingers or cantilevered arms of switches and the like. In addition to the more or less smooth surface which normally exists in the field of planar technology, these mechanical structures have a three-dimensional structural design and comprise cantilevered structures and buried cavities.
Up to now, it has been known to produce such cavities by wet-chemical etching or by connecting two disks which have been fully processed individually in advance. The techniques which may be used as a connection technique for connecting the individually processed disks are wafer bonding (silicon fusion bonding), anodic bonding or glueing. In any case, the individual disks are fully processed before they are connected in accordance with conventional methods, so that processing steps which may impair the mechanical structures need not be carried out afterwards.
When a wet-chemical undercutting of structures is carried out, the so-called xe2x80x9cstickingxe2x80x9d problem arises in the case of which the cantilevered structure will adhere to the neighbouring surface due to capillary forces occurring when the liquid dries; hence, the cantilevered structure will lose its movability. Small ditches, holes and gaps additionally cause problems when the structure in question is to be wetted with liquids (e.g. etching solutions, cleaning water, photoresist) and during the removal of these liquids, the problems being then caused e.g. by bubbles which may adhere to the structure in corners thereof. In the case of spinning, drops may remain, which will cause marks when they dry up. Cleaning by means of brushes is problematic as well, since the movable structures may break off during such cleaning processes. A clean surface of the structures is, however, necessary so that the production methods for producing the evaluation structures, such as a CMOS method, can be applied. Due to the risk of carrying over particles and contaminations, the CMOS ability may no longer be given in the case of open structures, i.e. certain sequences of process steps are not allowed in an CMOS line. In addition, when the chips comprised in the wafer are diced by means of a wafer saw, water is used as a rinsing liquid, which may penetrate into open cavities thus aggravating the particle and contamination problem.
It is additionally known to produce diaphragm-like structures making use of KOH back etching, when the electronic components on the front surface of a wafer have been finished. Due to the oblique etch edges occurring in the case of KOH etching, the integration level will, however, decrease substantially when this method is used, especially when a high number of micro-electromechanical components is produced from one wafer.
Various methods of producing semiconductor pressure sensors are additionally described in DE 3743080 A1.
DE-C-19543893 describes a method of aligning structures which are to be produced in a substrate, in the case of which a diaphragm-like structure is formed on top of a cavity. For this purpose, a closed cavity is first produced between two substrates by forming a recess in a masking layer on one of the substrates, whereupon the two substrates are interconnected via the masking layer and one of the substrates is thinned in a final step.
EP-A-639761 discloses a method of producing a differential pressure sensor in the case of which a substrate structure, in which a micromechanical element is formed, is connected to a glass substrate.
EP-A-702221 describes a method of producing a micromechanical structure and an integrated circuit in one component, the integrated circuit being produced in a first step making use of CMOS processes and the micromechanical structure, which consists of a cavity and a diaphragm, being implemented after the production of the integrated circuit.
It is the object of the present invention to provide a method of producing a micromechanical structure and a micromechanical element, which permit the use of conventional standard processes for producing evaluation circuits in the same substrate in which the micromechanical structure is formed, the method permitting a high yield to be achieved for the resultant micro-electromechanical elements and the micromechanical stucture being adapted to be used in a flexible manner.
This object is achieved by a method of producing a micromechanical structure for a micro-electromechanical element in the case of which a first intermediate layer, which is applied to a first main surface of a first semiconductor wafer, is structured in a first step so as to produce a recess. Subsequently, the first semiconductor wafer is connected via the first intermediate layer to a second semiconductor wafer in such a way that a hermetically sealed cavity is defined by the recess. Then, one of the wafers is thinned from a surface facing away from the first intermediate layer so as to produce a diaphragm-like structure on top of the cavity. At final version least one further intermediate layer is provided between the two semiconductor wavers which, prior to the connection of the two semiconductor wafers, is structured in such a way that the structure formed in said at least one further intermediate layer and the recess in said first intermediate layer define the cavity.
In the case of the micromechanical structure produced in this way, in which the cavity remains hermetically sealed after the thinning of one of the wafers, an integrated circuit can now be integrated in the thinned wafer making use of conventional standard semiconductor production processes. In the course of these processes, the cavity which, together with the diaphragm-like structure, defines the micromechanical element will always remain hermetically sealed so that the above-described problems entailed by the use of e.g. an CMOS method for producing an integrated circuit will not arise when the method according to the present invention is used. In addition, in view of the hermetically sealed cavity, the above-mentioned problems of removing the etching liquids from the cavities will not arise, since, due to the fact that the cavities are hermetically sealed, it is impossible that media, such as liquids, gases, solids and the like, penetrate into the respective cavity during the production of the integrated circuit. It follows that the method of producing a micromechanical structure according to the present invention permits micro-electromechanical elements, e.g. pressure sensors or controlled valves, to be produced by a reduced number of processing steps so that production at a reasonable price is made possible by the present invention.
According to the present invention, these advantages are achieved in that a hermetically sealed cavity is formed in an intermediate layer which is used for connecting two semiconductor wafers. According to preferred embodiments of the present invention, both semiconductor wafers have an intermediate layer applied thereto, one or both of these intermediate layers being structured so as to form the cavity after the connecting step; the intermediate layers are insulating or conductive layers which may consist e.g. of oxide (thermal or TEOS), polysilicon, nitride or metal. These intermediate layers can be interconnected by means of known connection methods, e.g. wafer bonding (silicon fusion bonding), anodic bonding or by means of an adhesive. Alternatively, a larger number of intermediate layers can be used between the wafers to be connected so as to permit the production of a cavity with areas of variable height.
The micromechanical structures produced according to the present invention are particularly suitable for use in pressure sensors, e.g. absolute pressure sensors or differential pressure sensors. In addition, the method according to the present invention can be used for producing micromechanical structures comprising a plurality of independent diaphragm areas so that a pressure sensor array can be produced in this way. The present invention can, however, also be used for providing micromechanical structures for use in other micro-electromechanical components, e.g. elastic sheets of valves or pumps, oscillating masses of acceleration sensors, movable fingers or cantilevers of micromechanical relays and the like.
Further developments of the present application are specified in the dependent claims.